Optical retarder

ABSTRACT

The present invention ( 1 ) includes a multi-focal plane display with two or more at least partially overlapping screens ( 2, 3 ) with a first order optical retarder ( 5 ) interposed therebetween. The invention removes the undersirable presence of coloured interference fringes whilst also providing a means of addressing the polarisation incompatibility between adjacent LCD display screens ( 2, 3 ).

TECHNICAL FIELD

[0001] The present invention applies generally to the field of opticalretarders and in particular to those suitable for use with multi-layeredviewing screens.

BACKGROUND ART

[0002] The benefits of multi-layered viewing screens, in particularthose utilising the technology described in the co-pending patentapplication Ser. Nos. NZ314566, NZ328074, NZ329130, PCT/NZ/98/00098 andPCT/NZ/99/00021 are gaining increasingly widespread recognition andacceptance due to their enhanced capabilities compared to conventionalsingle focal plane displays.

[0003] The manner in which human beings process visual information hasbeen the subject of extensive and prolonged research in an attempt tounderstand this complex process. The term preattentive processing hasbeen coined to denote the act of the subconscious mind in analysing andprocessing visual information which has not become the focus of theviewer's conscious awareness.

[0004] When viewing a large number of visual elements, certainvariations or properties in the visual characteristics of elements canlead to rapid detection by preattentive processing. This issignificantly faster than requiring a user to individually scan eachelement, scrutinising for the presence of the said properties. Exactlywhat properties lend themselves to preattentive processing has in itselfbeen the subject of substantial research. Colour, shape,three-dimensional visual clues, orientation, movement and depth have allbeen investigated to discern the germane visual features that triggereffective preattentive processing. Researchers such as Triesman [1985]conducted experiments using target and boundary detection in an attemptto classify preattentive features. Preattentive target detection wastested by determining whether a target element was present or absentwithin a field of background distractor elements. Boundary detectioninvolves attempting to detect the boundary formed by a group of targetelements with a unique visual feature set within distractors. It maybereadily visualised for example that a red circle would be immediatelydiscernible set amongst a number of blue circles. Equally, a circlewould be readily detectable if set amongst a number of square shapeddistractors. In order to test for preattentiveness, the number ofdistractors as seen is varied and if the search time required toidentify the targets remains constant, irrespective of the number ofdistractors, the search is said to be preattentive. Similar search timelimitations are used to classify boundary detection searches aspreattentive.

[0005] A widespread threshold time used to classify preattentiveness is200-250 msec as this only allows the user opportunity for a single‘look’ at a scene. This timeframe is insufficient for a human toconsciously decide to look at a different portion of the scene. Searchtasks such as those stated above maybe accomplished in less than 200msec, thus suggesting that the information in the display is beingprocessed in parallel unattendedly or pre-attentively.

[0006] However, if the target is composed of a conjunction of uniquefeatures, i.e. a conjoin search, then research shows that these may notbe detected preattentively. Using the above examples, if a target iscomprised for example, of a red circle set within distractors includingblue circles and red squares, it is not possible to detect the redcircle preattentively as all the distractors include one of the twounique features of the target.

[0007] Whilst the above example is based on a relatively simple visualscene, Enns and Rensink [1990] identified that targets given theappearance of being three dimensional objects can also be detectedpreattentively. Thus, for example a target represented by a perspectiveview of a cube shaded to indicate illumination from above would bepreattentively detectable amongst a plurality of distractor cubes shadedto imply illumination from a different direction. This illustrates animportant principle in that the relatively complex, high-level conceptof perceived three dimensionality may be processed preattentively by thesub-conscious mind. In comparison, if the constituent elements of theabove described cubes are re-orientated to remove the apparent threedimensionality, subjects cannot preattentively detect targets which havebeen inverted for example. Additional experimentation by Brown et al[1992] confirm that it is the three dimensional orientationcharacteristic which is preattentively detected. Nakaymyama andSilverman [1986] showed that motion and depth were preattentivecharacteristics and that furthermore, stereoscopic depth could be usedto overcome the effects of conjoin. This reinforced the work done byEnns Rensink in suggesting that high-level information is conceptuallybeing processed by the low-level visual system of the user. To test theeffects of depth, subjects were tasked with detecting targets ofdifferent binocular disparity relative to the distractors. Resultsshowed a constant response time irrespective of the increase indistractor numbers.

[0008] These experiments were followed by conjoin tasks whereby bluedistractors were placed on a front plane whilst red distractors werelocated on a rear plane and the target was either red on the front planeor blue on the rear plane for stereo colour (SC) conjoin tests, whilststereo and motion (SM) trials utilised distractors on the front planemoving up or on the back plane moving down with a target on either thefront plane moving down or on the back plane moving up.

[0009] Results showed the response time for SC and SM trials wereconstant and below the 250 msec threshold regardless of the number ofdistractors. The trials involved conjoin as the target did not possess afeature unique to all the distractors. However, it appeared theobservers were able to search each plane preattentively in turn withoutinterference from distractors in another plane.

[0010] This research was further reinforced by Melton and Scharff [1998]in a series of experiments in which a search task consisting of locatingan intermediate-sized target amongst large and small distractors testedthe serial nature of the search whereby the target was embedded in thesame plane as the distractors and the preattentive nature of the searchwhereby the target was placed in a separate depth plane to thedistractors.

[0011] The relative influence of the total number of distractors present(regardless of their depth) verses the number of distractors presentsolely in the depth plane of the target was also investigated. Theresults showed a number of interesting features including thesignificant modification of the response time resulting from the targetpresence or absence. In the target absence trials, the reaction times ofall the subjects displayed a direct correspondence to the number ofdistractors whilst the target present trials did not display any suchdependency. Furthermore, it was found that the reaction times ininstances where distractors were spread across multiple depths werefaster than for distractors located in a single depth plane.

[0012] Consequently, the use of a plurality of depth/focal planes as ameans of displaying information can enhance preattentive processing withenhanced reaction/assimilation times.

[0013] There are two main types of Liquid Crystal Displays used incomputer monitors, passive matrix and active matrix. Passive-matrixLiquid Crystal Displays use a simple grid to supply the charge to aparticular pixel on the display. Creating the grid starts with two glasslayers called substrates. One substrate is given columns and the otheris given rows made from a transparent conductive material. This isusually indium tin oxide. The rows or columns are connected tointegrated circuits that control when a charge is sent down a particularcolumn or row. The liquid crystal material is sandwiched between the twoglass substrates, and a polarizing film is added to the outer side ofeach substrate.

[0014] A pixel is defined as the smallest resolvable area of an image,either on a screen or stored in memory. Each pixel in a monochrome imagehas its own brightness, from 0 for black to the maximum value (e.g. 255for an eight-bit pixel) for white. In a colour image, each pixel has itsown brightness and colour, usually represented as a triple of red, greenand blue intensities. To turn on a pixel, the integrated circuit sends acharge down the correct column of one substrate and a ground activatedon the correct row of the other. The row and column intersect at thedesignated pixel and that delivers the voltage to untwist the liquidcrystals at that pixel.

[0015] The passive matrix system has significant drawbacks, notably slowresponse time and imprecise voltage control. Response time refers to theLiquid Crystal Displays ability to refresh the image displayed.Imprecise voltage control hinders the passive matrix's ability toinfluence only one pixel at a time. When voltage is applied to untwistone pixel, the pixels around it also partially untwist, which makesimages appear fuzzy and lacking in contrast.

[0016] Active-matrix Liquid Crystal Displays depend on thin filmtransistors (CIF). Thin film transistors are tiny switching transistorsand capacitors. They are arranged in a matrix on a glass substrate. Toaddress a particular pixel, the proper row is switched on, and then acharge is sent down the correct column. Since all of the other rows thatthe column intersects are turned off, only the capacitor at thedesignated pixel receives a charge. The capacitor is able to hold thecharge until the next refresh cycle. And if the amount of voltagesupplied to the crystal is carefully controlled, it can be made tountwist only enough to allow some light through. By doing this in veryexact, very small increments, Liquid Crystal Displays can create a greyscale. Most displays today offer 256 levels of brightness per pixel.

[0017] A Liquid Crystal Display that can show colours must have threesubpixels with red, green and blue colour filters to create each colourpixel. Through the careful control and variation of the voltage applied,the intensity of each subpixel can range over 256 shades. Combining thesubpixel produces a possible palette of 16.8 million colours (256 shadesof red ×256 shades of green ×256 shades of blue).

[0018] Liquid Crystal Displays employ several variations of liquidcrystal technology, including super twisted nematics, dual scan twistednematics, ferroelectric liquid crystal and surface stabilizedferroelectric liquid crystal. They can be lit using ambient light inwhich case they are termed as reflective, backlit and termed Tranmissive, or a combination of backlit and reflective and calledtransflective. There are also emissive technologies such as OrganicLight Emitting Diodes, and technologies which project an image directlyonto the back of the retina which are addressed in the same manner asLiquid Crystal Displays. These devices are described hereafter as LCDpanels.

[0019] In the case of a display comprising two or more overlappingparallel LCD panels, an inherent characteristic of using conventionallyconstructed LCD screens is that the polarisation of the light emanatingfrom the front of the rearward screen is mis-aligned with theorientation of rear polariser of the front screen.

[0020] Known techniques to overcome this drawback have to date involvedthe use of retarder films located between the two liquid crystaldisplays.

[0021] Optical retarders, also known as retardation plates, wave platesand phase shifters, may be considered as polarisation form converterswith close to a 100% efficiency. A retarder may be simply defined as atransrnisive material having two principle axes, slow and fast, whichresolves the incident beam into two orthogonally polarised componentsparallel to the slow and fast axes without appreciable alteration of theof the intensity or degree of polarisation. The component parallel tothe slow axis is retarded with respect to the beam component parallel tothe fast axis. The two components are then reconstructed to form asingle emergent beam with a specific polarisation form. The degree ofretardance/retardation denoting the extent to which the slow componentis retarded relative to the fast component is generally expressed interms of

[0022] a) linear displacement—the difference in the optical path lengthbetween the wave fronts of the two components, expressed in nanometers(nm);

[0023] b) fractional wavelength—the optical path length differenceexpressed as a fraction of a given wavelength, obtained by dividinglinear displacement values by a particular phase angle value orwavelength by 2π, e.g 280 nm/560 nm=½ wave retarder; and

[0024] c) phase angle—the phase difference between the wave fronts ofthe two component beams, expressed in degrees eg 90°, 180° or radians,{fraction (1/27)}π, π.

[0025] It can be thus seen that:

δ=Γ/λ.2π

[0026] where δ=the phase angle

[0027] Γ=the linear displacement

[0028] λ=the wavelength

[0029] Γ/λ=is the fractional wavelength.

[0030] If the thickness of the retarder produces a linear displacementless than the wavelength, the retarder produces a phase angle of lessthan π and is said to be of the first order. If the resultant phaseangle is between π and 2π then the retarder is said to be of the secondorder, if between 2π and 3π it is a third order retarder and so forth.The mean wavelength of the visible spectrum (560 nm) is used as thereference wavelength for optical retarders.

[0031] Correspondingly, a retarder may be employed as a polarisationform converter to rotate the output polarised light from the rear mostliquid crystal display of a multi-screened LCD unit through the requiredangle to align with the polarisation plane of the rear surface of thefront liquid crystal display. Polyesters such as polycarbonate are knownretarders with a low intrinsic cost, though they are difficult toproduce with sufficient chromatic uniformity to avoid the appearance ofcoloured ‘rainbow-like’ interference patterns when viewed betweencrossed polarisers. This is due (at least in part) to the thickness towhich such sheets of polycarbonate are available, which result in secondor higher order retarders.

[0032] In second, third or higher order retarders, the differentwavelengths of the spectrum constituents of white light are retarded toby the same linear displacement, but by different phase angles such thatpronounced coloured interference fringes result.

[0033] There are further complications with the manufacture of suchmulti-focal plane LCD displays. The fine regular structures formed bythe coloured filters and black pixel matrix on the alignment layers ofeach liquid crystal display produce a specific pattern in the lighttransmitted which, when combined with the corresponding pattern createdby the second liquid crystal display, causes an interference effect—i.e.moiré interference, degrading the resulting image seen by the viewer.

[0034] In order to eliminate these interference effects, a diffuser isinserted between the two liquid crystal displays. This may take the formof an individual layer/sheet or alternatively be formed by theapplication of a particular pattern or structure to the surface of theretarder. Chemical etching is a relatively cheap means of applying therequired pattern, though in practice it has been found deficient forproducing acceptable results in combination with a polyester orpolyester retarder.

[0035] Alternatives to chemical etching include embossing, impressing orcalendering of the said pattern by a holographically-recorded masteronto the surface of the polyester retarder, forming a random,non-periodic surface structure. These randomised structures may beconsidered as a plurality of micro lenslets diffusing incident light toeliminate moiré interference and colour defraction. This method ishowever significantly more expensive than conventional methods such aschemical etching. Further alternatives include specifically engineeredretarder films with no diffusive capability but these are also costlyand have chromatic uniformity problems.

[0036] It is also possible to assemble the front liquid crystal displaypanel with the polarising plane of the rearward surface aligned withthat of the front surface of the rear-most liquid crystal display.Unfortunately, this involves a large non-refundable engineering cost asit cannot be accommodated in the manufacture of conventional LCD unitsand thus requires production as custom units. In practice it is notpossible to rotate the polarisers on the forward display panel withoutchanging the rubbing axis on the glass as the contrast ratio of theimage would deteriorate. However, there would be no physical indicationof the rubbed orientation of an LCD mother glass (as the processliterally involves rubbing the polyimide layer on the glass with arotating velvet cloth) without labelling and this would causesignificant disruption to the manufacturing process.

[0037] By contrast, use of a retarder enables the requisite polarizationorientation change to be discerned by examining the polarization/glassfinish to aclimate the retarder adhesive. This is clearly visible by theunaided eye and is one of the last production stages, thus reducingpotential risk.

[0038] It is also possible to utilise a third party (i.e., not theoriginal manufacturer) to realign the respective polarising screensthough this is also expensive and runs the risk of damaging the displaypanels. Damage can occur during numerous steps in such a third partyprocedure, including any or all of the following;

[0039] 1. removing the LCD panel from its surround:—possible damage toTAB drivers or the glass;

[0040] 2. removing the polarizer:—heating of the polariser is requiredto reduce its adhesion and can damage the glass, damage individualpixels from excessive pressure, and the liquid crystals may beoverheated;

[0041] 3. misalignment of the new polariser;

[0042] 4. replacing the panel in the original packaging—again causingpossible damage to tab boards or the glass; and

[0043] 5. electrical static damage at any point of the procedure.

[0044] Furthermore, some or all of the interstitial optical elementslocated between the display layers (i.e. the LCD panels) may change theoptical path length of light incident on the second (or successive)screen having passed through the first display. This alteration in pathlength leads to chromatic aberrations that require correction to ensurea clear display image.

[0045] Interstitial elements which may introduce such optical pathlength changes include:

[0046] Air;

[0047] nitrogen, or any other inert gas;

[0048] a selective diffusion layer;

[0049] Polymer Dispersed Liquid Crystal;

[0050] Ferroelectric Liquid Crystal;

[0051] Liquid Crystal between random homogeneous alignment layers;

[0052] Acrylic, Polycarbonate, Polyester;

[0053] Glass;

[0054] Antireflective coating;

[0055] Optical cement;

[0056] Diffusive film;

[0057] Holographic diffusion film; and

[0058] any other filter for removing moiré interference

[0059] Thus, there is the combined need to cost-effectively re-align thepolarisation between successive LCD panels, whilst avoid chromaticaberrations such as coloured interference fringes present with the useof existing retarder such as polycarbonate.

[0060] All references, including any patents or patent applicationscited in this specification are hereby incorporated by reference. Noadmission is made that any reference constitutes prior art. Thediscussion of the references states what their authors assert, and theapplicants reserve the right to challenge the accuracy and pertinency ofthe cited documents. It will be clearly understood that, although anumber of prior art publications are referred to herein, this referencedoes not constitute an admission that any of these documents form partof the common general knowledge in the art, in New Zealand or in anyother country.

[0061] It is acknowledged that the term ‘comprise’ may, under varyingjurisdictions, be attributed with either an exclusive or an inclusivemeaning. For the purpose of this specification, and unless otherwisenoted, the term ‘comprise’ shall have an inclusive meaning—i.e. that itwill be taken to mean an inclusion of not only the listed components itdirectly references, but also other non-specified components orelements. This rationale will also be used when the term ‘comprised’ or‘comprising’ is used in relation to one or more steps in a method orprocess.

[0062] It is an object of the present invention to address the foregoingproblems or at least to provide the public with a useful choice.

[0063] Further aspects and advantages of the present invention willbecome apparent from the ensuing description which is given by way ofexample only.

DISCLOSURE OF INVENTION

[0064] According to one aspect of the present invention there isprovided a multi-focal plane display including at least two at leastpartially overlapping display surfaces having a first order opticalretarder interposed between at least two said screens.

[0065] A first order optical retarder produces a phase angledisplacement or retardation of less than or equal to that of theincident wavelength. Furthermore, it has been found that a first orderretarder does not produce discernible coloured interference fringes whenused in said displays.

[0066] Suitable materials for production of first order retarders havehitherto suffered from significant drawbacks such as instabilityunderexposure to bright lights and/or ageing, discoloration over time,manufacturing expense, brittleness and so forth.

[0067] Thus, according to a further aspect of the present invention,there is provided a display as hereinbefore described, wherein saidfirst order retarder is a material with the optical properties of abiaxial polypropylene.

[0068] Preferably, the said optical properties include those of adiffuser.

[0069] The diffuser may be either formed as a separate layer distinctfrom said retarder or diffusive properties may be applied to the surfaceof the retarder itself.

[0070] According to a further embodiment, said diffusive effects of thediffuser are formed by a means selected from the group comprisingchemical etching; embossing; impressing: or calendering a random,non-periodic surface structure onto the diffuser surface.

[0071] The ideal separation of the said diffuser from the surface of thedisplay surface is a trade off between image clarity (decrease withseparation) and diffusion of the moiré effects. The separation of thediffusive layer from the display surface can be controlled by usingadhesive of various thickness, to attach the diffuser to the displaysurface. This is applicable for both the use of a separate distinctdiffuser or one integrally formed with, or attached to the saidretarder.

[0072] Thus, according to a further aspect of the present invention, thesaid diffuser is adhered to said display by adhesive of a predeterminedthickness.

[0073] In a display as described herein, used with visible light with amean wavelength of 560 nm, said first order retarder has a phasedifference of less than or equal to 560 nm.

[0074] Thus, according to a further aspect of the present invention,said retarder causes a phase angle retardation of less than or equal toone wavelength of light incident on said display. This is may bealternatively expressed as a linear displacement of less than or equalto 560 nm of said incident light.

[0075] The biaxial polypropylene is preferably formed as clear flexiblefilm, though may conceivably be formed as a film, lacquer or coating.

[0076] According to another aspect of the present invention there isprovided a method of manufacturing a multi-focal plane display includingpositioning a first order optical retarder between at least twopartially overlapping display surfaces.

[0077] According to another aspect of the present invention there isprovided a biaxial polypropylene layer adapted for use in an opticalsystem.

[0078] Said optical system need not be restricted to multi-focal planedisplays as described above, but includes any optical system capable ofutilising the said optical properties of biaxial polypropylene, and inparticular, those of a retarder.

[0079] However, to date, biaxial polypropylene has not been employed forits optical properties, and in particular those of retardation. It hasbeen found that replacing known retarders—such as polycarbonate inmulti-layer displays by film of biaxial polypropylene that unexpectedlyadvantageous results are obtained in comparison to the prior art.

[0080] The multi focal plane displays are preferably formed from liquidcrystal panels, though it will be appreciated that other forms ofoptically active display elements may be used and are thus incorporatedwithin the scope of the present invention.

BRIEF DESCRIPTION OF DRAWINGS

[0081] Further aspects of the present invention will become apparentfrom the following description which is given by way of example only andwith reference to the accompanying drawings in which:

[0082]FIG. 1 shows a diagrammatic representation of a multi-focal planedisplay in accordance with one embodiment of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

[0083] A preferred embodiment of the present invention is shown in FIG.1 incorporated in a dual screen display (1) of known type. The display(1) includes two overlapping, parallel liquid crystal display screens(2, 3) upon which information and/or images may be displayed by avariety of known means. In the preferred embodiment shown in simplifiedform in FIG. 1, a back light (4) is placed behind the rear LCD screen(2) to provide illumination for the images shown on one or both LCDscreens (2, 3).

[0084] To minimise costs, both LCD screens (2, 3) are constructed inaccordance with conventional manufacturing techniques, i.e. crossedpolarising filters are located on the front and rear surface of eachliquid crystal active element. A consequence of the characteristicoperating mechanism of liquid crystal displays is that the planepolarisation of the light emerging from the front surface of the rearLCD screen (2) is crossed with respect to the polarisation plane of therear surface of the front LCD screen (3).

[0085] To rotate the emergent light from the rear screen (2) by therequired angle to align with the rear polarisation filter of the frontLCD screen (3), an optical retarder (5) is placed between the LCDscreens (2, 3). Whilst in theory, the retarder (5) may be placedanywhere between the screens (2, 3), the use of prior art retarders suchas polyester necessitates a location adjacent the front of the rear LCDscreen (2). This is primarily due to the need for a diffusive pattern tobe applied to retarders to avoid interference effects degrading theresultant display (1) image. Interference patterns are generated by boththe Moiré effect,—i.e., interference caused by slight period disparitiesbetween the structured surface on the LCD screens (2, 3), and theeffects of chromatic separation of white polarised light into ‘rainbow’coloured fringes. Diffusing the light is therefore used to deregulatethe interference patterns generated.

[0086] It has been found in practice that chemical etching of thediffusion pattern on polyester does not provide sufficient control ofthe colour interference patterns. The main alternative to chemicaletching involves embossing a holographically recorded master with arandomised surface structure onto the polycarbonate retarder surface.This process is however significantly more expensive than chemicaletching.

[0087] Prior art alternatives also include custom manufactured LCDscreens constructed with the rear polarising filter of the front LCDscreen (3) already aligned with the rear polariser of the front LCDscreen (3). This re-alignment may also be undertaken after manufactureby a third party, albeit with a significant risk of damage to the LCDscreens. Both means of re-alignment are prohibitively expensive.

[0088] The present invention addresses this need by use of a biaxialpolypropylene film as a first order retarder (5) located between the LCDscreens (2, 3). Biaxial polypropylene available direct from commercialstationery outlets has been found to produce surprisingly good resultsin terms of optical performance in addition to the obvious cost andavailability benefits. A brightness gain of 1.96 has been measured incomparison to existing polyester retarders. Furthermore, biaxialpolypropylene of sufficient thickness to form a first order retardereliminates the colour interference effects whilst also permitting theuse of chemically etched diffusion pattern to eliminate the Moiréinterference effect without loss of image quality.

[0089] As described earlier, the degree of retardance/retardation isgenerally expressed in terms of

[0090] d) linear displacement—the difference in the optical path lengthbetween the wave fronts of the two components, expressed in nanometers(nm);

[0091] e) fractional wavelength—the optical path length differenceexpressed as a fraction of a given wavelength, obtained by dividinglinear displacement values by a particular phase angle value orwavelength by 2π; and

[0092] f) phase angle—the phase difference between the wave fronts ofthe two component beams;

[0093] It and thus, it can be thus seen that:

δ=Γ/λ.2π

[0094] where δ=the phase angle

[0095] Γ=the linear displacement

[0096] λ=the wavelength

[0097] Γ/λ=is the fractional wavelength.

[0098] As biaxial polypropylene may readily be produced as thin flexibledurable sheets, sufficiently thin to produce a linear displacement lessthan one wavelength of visible light, i.e, the retarder produces a phaseangle of less than a, it is said to be of the first order. Prior artretarders used in this application produced resultant phase anglesbetween 2π and 2π, or between 2π and 3π, i.e. second order, or thirdorder retarders respectively.

[0099] The chemically etched diffusion pattern may be applied to adiffuser in the form of sheet of acrylic (6) or similar placed betweenthe LCD screens (2, 3). The biaxial polypropylene also providessufficient chromatic uniformity that the retarder (5) can be placed atany point between the LCD screens (2, 3).

[0100] According to further embodiments (not shown) the diffuser (6) maybe either formed as a separate layer distinct from said retarder (5) ordiffusive properties may be applied to the surface of the retarder (5)itself.

[0101] The said diffusive effects of the diffuser (6) may be formed bychemical etching; embossing; impressing: or calendering a random,non-periodic surface structure onto the diffuser surface.

[0102] The ideal separation of the said diffuser (6) from the surface ofthe display (3) surface is a trade off between image clarity (decreasewith separation) and diffusion of the moiré effects (increasing withseparation). This separation can be controlled by using adhesive (notshown) of a predetermined thickness, to attach the diffuser (6) to thedisplay (3) surface. This is applicable for both embodiments using of aseparate distinct diffuser (6) or one integrally formed with, orattached to the said retarder (5).

[0103] It is envisaged that the biaxial polypropylene film thickness andvariations in the manufacturing processes and/or constituents may affectsome optical properties including the difference in refractive index foreach polarisation axis, different frequencies and temperature. However,according to tests to date, biaxial polypropylene exhibits achromaticretarding properties.

[0104] It will be appreciated that although the preferred embodiment hasbeen described with reference to a dual-screen liquid crystal display,the invention is not limited to same. It will also be apparent to thoseskilled in the art that the invention may be equally applicable to otheroptical systems benefiting from the said properties of a biaxialpolypropylene retarder.

[0105] Furthermore, it will be understood that other materials may beused as retarders in such multi-focal plane displays provided theyprovide the first order retardive properties of biaxial polypropylene.

[0106] Aspects of the present invention have been described by way ofexample only and it should be appreciated that modifications andadditions may be made thereto without departing from the scope thereof.

1. A multi-focal plane display including at least two at least partiallyoverlapping display surfaces having a first order optical retarderinterposed between at least two said screens.
 2. A display as claimed inclaim 1, wherein said first order retarder is a material with theoptical properties of a biaxial polypropylene.
 3. A display as claimedin claim 1 or claim 2, wherein said optical properties include those ofa diffuser.
 4. A display as claimed in claim 1 or claim 2, furthercomprising a diffuser formed as a distinct layer from said retarder. 5.A display as claimed in claim 4, wherein said diffuser is adhered tosaid display by adhesive of a predetermined thickness.
 6. A display asclaimed in any one of claims 1-5, wherein said biaxial polypropylene isformed as a clear flexible film.
 7. A display as claimed in any one ofclaims 1-5, wherein said biaxial polypropylene is formed as a lacquer orcoating.
 8. A display as claimed in any one of the preceding claims,wherein said display surfaces are formed from liquid crystal displaypanels.
 9. A display as claimed in any one of the preceding claims,wherein said retarder causes a phase angle retardation of less than orequal to one wavelength of light incident on said display.
 10. A displayas claimed in claim 9, wherein said retarder causes a lineardisplacement of less than or equal to 560 nm of said incident light. 11.A display as claimed in any one of the preceding claims, wherein saiddiffusive effects are formed by a means selected from the groupcomprising chemical etching; embossing; impressing: or calendering arandom, non-periodic surface structure onto the diffuser surface.
 12. Amethod of manufacturing a multi-focal plane display as claimed in anyone of claims 1-11, including the step of positioning a first orderoptical retarder between at least two partially overlapping displaysurfaces.
 13. The method as claimed in claim 12 wherein said first orderoptical retarder sheet is formed from biaxial polypropylene.
 14. Abiaxial polypropylene layer adapted for use in an optical system as afirst order retarder.
 15. A multi focal plane display substantially ashereinbefore described, with reference to, and as shown in theaccompanying drawing.
 16. A method of manufacturing a multi-focal planedisplay substantially as hereinbefore described, with reference to, andas shown in the accompanying drawing.